October 9, 2006

Snow cover and sea ice extent have long been thought of as monitors of climate change, and so it is no surprise that the global warming crusade has promoted any signal of glacial decline as clear evidence of global warming. The simplicity of the concept that “warming means less ice” makes a decline in snow and ice an appealing piece of ammunition for their cause. However, wielding this with a narrow view of the issue makes it dangerous ammunition, or useless, as appears to be the case for one set of tropical glaciers that are widely referenced in the global warming debate.

In a recent article in Geophysical Research Letters entitled “Kilimanjaro glaciers: recent areal extent from satellite data and new interpretation of observed 20th century retreat rates,” the glaciers of Africa’s highest mountain are in effect removed from the debate of climate change in the 20th century. The work of a team led by Nicolas Cullen of the Tropical Glaciology Group at the University of Innsbruck is significant because drastic retreats in tropical glaciers during the mid- to late-20th century have attracted broad attention among global warming alarmists.

What many climate change arguments tend to omit is the fact that periods of glacial advance and retreat are not merely a reflection of air temperature trends, as a change in precipitation, for example, is equally important. In the tropics, glaciers are particularly sensitive to humidity and cloudiness, which control the amount of incident solar radiation. Furthermore, Cullen et al. point out that the glaciers of Kilimanjaro are above the mean freezing level, meaning that it is “difficult to suggest that air temperature changes alone are responsible for glacier recession on Kilimanjaro.” To re-evaluate possible causes of glacier retreat on Kilimanjaro, Cullen et al. employed recent high spatial resolution satellite images of the mountain to construct a new detailed map of the of the ice bodies. They compared the new data to long-term variations in ice extent to assess its retreat in the context of 20th century changes in air temperature, atmospheric moisture, and precipitation in East Africa.

The group of researchers found that glacial retreat during the 20th century was profound, as their work shows that only 21% of the 1912 ice cover on Kilimanjaro existed in 2003 when the satellite images were taken. However, the highest recession rates occurred in the first part of the 20th century, while the recession rate over the last 15-year interval (1989-2003) was smaller than in any of the other defined intervals in the study period of 1912-2003. Obviously, this is counterintuitive to the idea that dramatic warming within Earth’s climate has occurred over the past several decades. Given this curious finding, Cullen et al. set out to interpret the findings in the context of 20th century climate change.

The ice bodies of Kilimanjaro were stratified into two types of glaciers – plateau (elevation ≥ 5700 m) and slope (< 5700 m) – to help differentiate by physical features such as shape, slope, thickness, and bed shape. Characterized by name, plateau glaciers are tabular shaped ice bodies that rest stably on flat surfaces. In contrast, slope glaciers are found on steeper surfaces and possess some downward movement. The retreat of all plateau glaciers was found to be continuous and linear since 1912, while slope ice bodies experienced a rather rapid recession between 1912 and 1953, followed by a decreasing rate of retreat. The constant retreat of the plateau glaciers does not indicate that climate fluctuations during the 20th century affected their demise. Instead, solar radiation incident on vertical walls of the glaciers produces irreversible melting despite air temperatures that remain below freezing. The mechanisms responsible for vertical wall development on plateau glaciers are not well understood. Cullen et al. conclude that the demise of the plateau glaciers of Kilimanjaro is unavoidable given their geometry, and that any recent change in climate has had no significant impact. Thus, the plateau glaciers of Kilimanjaro are excused from the global warming debate.

Unlike with plateau glaciers, Cullen et al. believe that slope glaciers have a much shorter adjustment time to changes in climate – on the order of a few years. The research group believes that the rapid recession in the early part of the 20th century (1912-1953) indicates that the glaciers were wildly out of equilibrium in responding to a prior shift in climate during the 19th century. Cullen et al. contend that the momentum of the shift has left the slope glaciers of Kilimanjaro out of equilibrium still today. Their most significant point here is that there is no evidence of any atmospheric warming in the 20th century at the altitude of the glaciers that might be responsible for trends toward the demise of this brand of glaciers on Kilimanjaro. Joining plateau glaciers, the slope glaciers of Kilimanjaro are excused from the global warming debate.

Cullen et al. conclude that “Rather than changes in 20th century climate being responsible for their demise, glaciers on Kilimanjaro appear to be remnants of a past climate that was once able to sustain them.” Should a debate on a mysterious climate shift of the late 19th century break-out, the glaciers of Kilimanjaro might be useful. As for debate over 20th century climate change, the glaciers of Kilimanjaro appear to be on ice.